The present invention relates to a radioactive ray image intensifier and, more particularly, to an improvement in contrast property in an X-ray image intensifier.
X-ray image intensifiers are widely utilized as an aid in the diagnosis of diseases in the medical field and for non-destructive testing in the industrial field.
An X-ray image intensifier has a configuration shown in FIG. 1. An evacuated envelope 1 is maintained at a high vacuum pressure and consists of an X-ray input window 2 of a thin metal plate, a body 5 mainly of a glass material, and a glass output window 4. An input screen 6, a focusing electrode 7, an anode 8 and an output screen 9 are disposed in the evacuated envelope 1. The input screen 6 comprises an aluminum substrate 10, a phosphor layer 11 formed thereon, and a photoemissive layer 12 formed on the layer 11. The output screen 9 comprises a glass substrate 13, a phosphor layer 14 formed thereon, and a metal back layer 15 formed on the layer 14.
X-rays emitted from an X-ray source 20 and transmitted through an object (not shown) pass through the X-ray input window 2 and are converted into visible light by the phosphor layer 11 formed on the input screen 6. Furthermore, the visible light is converted into photo electrons by the photoemissive layer 12 formed on the layer 11. The photo electrons are accelerated and focused by the focusing electrode 7 and the anode 8 constituting an electron lens, and are converted into visible light again in the output screen 9, thus forming a visible image.
The visible image is observed through the output window 4 or a television set, or is observed as a photograph, thus aiding in the diagnosis of disease. Therefore, in order to facilitate an accurate diagnosis, the image appearing on the output screen 9 must be of high quality.
When an X-ray image intensifier is used, it is arranged in an X-ray diagnosis apparatus. In this case, the image intensifier itself is fixed in a container (housing) 16 by a supporting mechanism 17. A magnetic shield 18 for reducing the effect of an external magnetic field and lead plates 19a and 19b for preventing X-ray leakage are attached to an inner surface of the container 16. The amount of X-ray leakage allowed in an image intensifier is regulated by the Welfare Ministry. For this reason, a lead plate which is thick enough to satisfy this regulation (e.g., 1.5 mm or more) must be attached. The shape and arrangement of electrodes in an electron lens system in the image intensifier are determined by computer calculations. The input screen 6 has a curved shape. An effective area of the screen 6 for receiving X-rays corresponds to a region indicated by A in FIG. 1. Note that a diverging angle of X-rays corresponding to the region A is indicated by a in FIG. 1. In this case, light emitted by the phosphor layer outside the effective irradiation area A is not received by the electron lens system.
X-rays emitted from an X-ray tube 20 at a constant diverging angle c toward the image intensifier are incident on an input portion region of the container 16 defined by the lead plate 19b attached to an inner wall of an input portion thereof. A diverging angle corresponding to this region is b. These angles a, b and c satisfy a relation a &lt;b &lt;c. When the X-rays are irradiated on an object, X-ray scattering or diffraction (reflection) occurs. In other words, an X-ray component of a diverging angle between the angles a and b is irradiated on the input portion, thereby generating a scattered X-ray, and an X-ray component of a diverging angle larger than the angle b is irradiated on the lead plate 19b, thereby also generating a scattered X-ray. These X-ray components are transmitted through the input window of the thin metal plate having a good X-ray transmission property, thus exciting the phosphor layer 11 of the effective irradiation region A. This results in degradation of contrast property in the X-ray image intensifier.
The effective irradiation region A of the input screen and the diverging angles a, b and c vary in accordance with a geometrical arrangement of the X-ray source, e.g., the X-ray tube 20 and a structure of the X-ray image intensifier. For this reason, an area of the region which generates the scattered or reflected X-ray by irradiation of an X-ray component of a diverging angle larger than the angle a is slightly varied. In this case, the scattered or reflected X-ray amount is determined by a material of a peripheral member in the region on which such an X-ray component is irradiated, e.g., the peripheral member of the input portion of the container 16 or the input window. Note that the container 16 is generally formed of an aluminum alloy and the input window is formed of aluminum, aluminum alloy, stainless steel, glass or the like.
In an image pickup tube utilizing light, an optical system such as a lens is utilized for converging light. For this reason, an incident angle of light is large. On the other hand, an X-ray incident upon the X-ray image intensifier is an X-ray cone having a directivity emitted from a point X-ray source. Therefore, since the X-ray has a high energy and is irradiated at a small incident angle, it is easily subject to scattering or reflection described above on a plane of incidence.
In recent X-ray image intensifiers, in order to further aid diagnosis of minute portions of an object, demand has arisen for an improvement in image quality by improving contrast property. Also, in order to use a diagnostic apparatus using digital image processing, a good contrast property is required.
Therefore, inconveniences in an X-ray image intensifier apparatus caused by scattering or reflection of an incident X-ray must be reduced in addition to an improvement in characteristics of the image intensifier itself.